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Halloysite tubule nanoreactors in industrial and agricultural applications
Elshad Abdullayev1
and Yuri Lvov2
1....
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the outside diameter of the tubes varies from 50 to 200 nm, and the diameter of the internal lumen ranges from ...
3	
  
	
  
Recently, it has been explicitly shown that controlled release of corrosion inhibitors plays critical role in
d...
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solution resumed the original release rate of the benzotriazole. With this one can switch on release of the
enc...
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Polymer-clay nanocomposites allow increased tensile and flexural strength [25]
, improved thermal resistance
an...
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Halloysite nanotubes have been effectively utilized as nucleating agents for several polymers like
polyethylene...
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Figure10. Mechanical properties of pure paint and halloysite paint nanocomposite. (a) Stress-strain relationshi...
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products were accumulated underneath the paint on strips coated with ordinary paint while strip coated with
hal...
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Figure 13. Scanning Electron Micrograph of the concrete prepared by type I Portland cement; control sample (lef...
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Figure 14. Pyrolysis of cottonseed oil and vacuum gasoil mixture on various catalysts. Variation of (a) gasoli...
11	
  
	
  
was extremely efficient in decomposing phosphine gas into yellow phosphorus, resulting in 99% conversion at
at...
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6. Summary and outlook
Halloysite nanotubes, having tubular shape with high aspect ratio and 50-100 nanometers...
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22. S. Konnova, I. Sharipova, T. Demina, Y. Osin, D. Yarullina, P. Zelenikhin, D. Ishmuchametova, O.
Ilinskaya...
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47. Y. M. Lvov, D. G. Shchukin, H. Mohwald, R. R. Price, “Halloysite Clay Nanotubes for Controlled Release
of ...
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Applied Minerals Dragonite(TM) Halloysite nanotubes as nanoreactors in industrial and agricultural applications

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Halloysite tubule nanoreactors in industrial and agricultural applications

1. Introduction

Halloysite is a two-layered aluminosilicate which has a predominantly hollow tubular structure in the submicron range and is chemically similar to kaolinite [1]. It is mined commercially from natural deposits in USA, New Zealand, China, Turkey, and Malaysia [2]. Dragon mine in Utah has exceptionally pure halloysite with nanotubes exceeding 99 %[3]. These minerals are formed from kaolinite over millions of years due to the hydrothermal processes [4]. Layers are rolled into tubes due to the strain caused by lattice mismatch between adjacent silicone dioxide and aluminum oxide sheets [1-3].

Halloysite was discovered in 18th century and named in honor of Omalius d’Halloy who analyzed the mineral [5]. In early years it was very difficult to distinguish halloysite from other minerals, particularly from kaolinite. However, X-ray analysis has shown that it has unique crystalline structure [6]. Halloysite has been extensively utilized as a raw material for ceramics industry, especially for the manufacture of porcelain, and bone china [1,7]. Having nanotubular structure, halloysite particles can potentially be applied in several fields of nanotechnology. These multilayer tubes are commonly used for plastic composites, in controlled release applications [8-15], and may be coated with metal by electroless plating to make conductive fillers [16]. Due to their porous structure and high catalytic activity, halloysite particles can be used in remediation of acid mine drainage, petroleum conversion in refining industries, as well as in the separation of liquids and gaseous mixtures. This paper summarizes the structure, physicochemical properties and major application areas of halloysite nanotubes.

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Applied Minerals Dragonite(TM) Halloysite nanotubes as nanoreactors in industrial and agricultural applications

  1. 1. 1     Halloysite tubule nanoreactors in industrial and agricultural applications Elshad Abdullayev1 and Yuri Lvov2 1. Applied Minerals Inc, 1200 Silver city road, Eureka, UT 84628 2. Louisiana Tech University, 911 Hergot avenue, Ruston, LA 71272 1. Introduction Halloysite is a two-layered aluminosilicate which has a predominantly hollow tubular structure in the submicron range and is chemically similar to kaolinite [1] . It is mined commercially from natural deposits in USA, New Zealand, China, Turkey, and Malaysia [2] . Dragon mine in Utah has exceptionally pure halloysite with nanotubes exceeding 99 %[3] . These minerals are formed from kaolinite over millions of years due to the hydrothermal processes [4] . Layers are rolled into tubes due to the strain caused by lattice mismatch between adjacent silicone dioxide and aluminum oxide sheets [1-3] . Halloysite was discovered in 18th century and named in honor of Omalius d’Halloy who analyzed the mineral [5] . In early years it was very difficult to distinguish halloysite from other minerals, particularly from kaolinite. However, X-ray analysis has shown that it has unique crystalline structure [6] . Halloysite has been extensively utilized as a raw material for ceramics industry, especially for the manufacture of porcelain, and bone china [1,7] . Having nanotubular structure, halloysite particles can potentially be applied in several fields of nanotechnology. These multilayer tubes are commonly used for plastic composites, in controlled release applications [8-15] , and may be coated with metal by electroless plating to make conductive fillers [16] . Due to their porous structure and high catalytic activity, halloysite particles can be used in remediation of acid mine drainage, petroleum conversion in refining industries, as well as in the separation of liquids and gaseous mixtures. This paper summarizes the structure, physicochemical properties and major application areas of halloysite nanotubes. 2. Halloysite structure Halloysite occurs in nature as a hydrated mineral consisting from rolled alumosilicate sheets that has the ideal formula of Al2Si2O5(OH)4.nH2O , which is similar to kaolinite except for the presence of an additional water monolayer between the adjacent layers. When n = 2 the mineral is called halloysite – 10 Å due to its layer periodicity of 10 Å. Heating halloysite – 10 Å irreversibly dehydrates it to halloysite – 7 Å with n=0 [1 - 5] . Dehydrated halloysite has a formula of Al2Si2O5(OH)4 with interlayer spacing of about 7.2 Å [1] . Pure halloysite is a white mineral and can easily be processed to obtain fine powder (Fig.1). However, in some cases the mineral is colored from yellowish to brown and sometimes greenish depending on the deposit. The reason for these colors are trace amounts of the metal ions such as Fe+3 , Cr+3 , Ti+4 , etc [1] that substitute Al+3 or Si+4 in the halloysite mineral. Scanning electron microscope (SEM) images of the halloysite samples clearly indicate that they have tubular nature. Figure 1. (a) Raw halloysite mineral from Dragon mine (Applied Minerals, Inc) and (b) SEM image from the rock, showing over 99 % nanotube content. Reprinted with permission from Ref. 3 Copyright 2013 Royal Society of Chemistry Halloysite layer is composed of the bilayers of aluminum and silicon oxides (Fig.2a). Dimensions of the halloysite tubes vary depending on deposit, i.e. each deposit has halloysite with certain diameter and length. Overall,
  2. 2. 2     the outside diameter of the tubes varies from 50 to 200 nm, and the diameter of the internal lumen ranges from 10 to 40 nm [1] . The lengths of the tubules are in the range 0.5 – 1.5 µm [2,14,15] . One of the specialties of the halloysite is the different surface chemical properties at the inner and outer sides of the tubes [14-17] (Fig.2a). The silica layer is relevant to the outer surface of tube, while the alumina layer is relevant to the inner lumen surface. Aluminum and silicon oxides have different dielectric and ionization properties, which is evident from the observations of electrical zeta-potentials of these oxide nanocolloids in water. The first one (alumina) has positive charge up to the pH value of the solution 8.5, while the other one (silica) is negative above pH 1.5 (Fig. 2b). The ability to have different charges at the inner and outer parts of the halloysite nanotubes allows for the selective loading of negatively charged molecules inside the nanotubes [8-15] . Figure 2. (a) Schematic representation of the halloysite tubular structure and wall chemistry. (b) Variation of the silica and alumina surface potentials by pH of the solution. Reprinted with permission from Ref. 3 Copyright 2013 Royal Society of Chemistry 3. Nanotubes for loading and controlled release Empty lumen of the halloysite nanotubes makes it an effective container for loading with active chemical agents. It has been loaded with big variety of organic and inorganic substances including corrosion inhibitors, biocides, metals and salts to prepare functional polymer composites. In Fig. 3 TEM images of halloysite nanotubes loaded with metallic silver nanoparticles and nanorods are presented. Silver was loaded into halloysite using silver acetate solution in water. Tubule lumen is filled with silver, indicating the high efficiency of the loading. High resolution TEM and XRD analysis indicated crystalline structure of the loaded silver nanorods[13] . Fig. 3. TEM images of the halloysite samples loaded with silver. Reprinted with permission from Ref. 13 Copyright 2011 American Chemical Society.
  3. 3. 3     Recently, it has been explicitly shown that controlled release of corrosion inhibitors plays critical role in delaying the rusting of steel in salty water solution[18] . Self-healing epoxy composites have been demonstrated by encapsulating diglycidyl ether of bisphenol A within halloysite lumen. Self-healing effect has been achieved by leaking the loaded bisphenol monomer at the cracks and cross linking with amine curing agent[19,20] . Figure 4. Brilliant green dissolution curve in water (squares and solid line) and sustained release from halloysite (circles and dashed line). Reprinted with permission from Ref. 3 Copyright 2013 Royal Society of Chemistry Common antiseptic agent, brilliant green, released in water within 5 minutes while only 80 % was released from halloysite within 2 hours (Fig. 4). 95 % release of brilliant green is achieved within 30 hours which is 200 times slower than its dissolution rate without encapsulation. Release time of many proteins from clay nanotubes is rather slow and lasts 50-500 hours. This may be due to their lower diffusion coefficient, as well as to stronger protein adsorption to the lumen walls (in all cases we washed out externally adsorbed proteins)[3,15] . Proteins have many OH, NH2 and other functional groups that interact with halloysite surface OH groups through hydrogen bonding or Van der Waals interaction[14] . 3.1 Synthesis of release stoppers Better encapsulation and control over release rate has been achieved by synthesis of artificial caps at halloysite tube endings. End caps are synthesized by interaction of leaking agent-A and complex forming agent - B (from external solution). This approach has been effective in encapsulation of triazole and imidazole derivatives. These substances form 2-D polymer film while interacting with transition metal ions such as Cu(II) and Fe(II). The film clots the tube endings and loaded agents become entrapped into the tube lumen[11,12] . Figure 5. Illustration of stopper formation at halloysite tube endings by interaction of leaking benzotriazole at Cu (II) ions. Reprinted with permission from Ref. 12 Copyright 2009 American Chemical Society Controllable release of benzotriazole loaded into the tubes was achieved by exposing the dispersion of loaded nanotubes to the solution containing Cu (II) ions. In the case of original halloysite (blank) benzotriazole simply diffuses out from the tube openings while in the presence of Cu(II) ions leaking is retarded due to the Cu-BTA film formation (Fig. 5). Benzotriazole release from such closed tubes occurs through much smaller pores within the film. Thickness of the film depends on concentration of the Cu(II) ions in the solution, i.e. higher concentration of the Cu(II) ions leads to better encapsulation of the benzotriazole[11] . Decomposition of the tube end caps with ammonia
  4. 4. 4     solution resumed the original release rate of the benzotriazole. With this one can switch on release of the encapsulated materials[12] . 3.2 Halloysite Lumen Enlargement Optimal loading efficiency of 5-10% by weight is easily achievable with current industrially available halloysite nanotubes. Lumen size enlargement allows more space for internal loading of active agents within tubular lumen. This has been achieved by selective etching of aluminum oxide layers from halloysite sheets by sulfuric acid (Fig.6)[21] . Figure 6. (a) Schematic of halloysite lumen etching. TEM images of (b) original and (c) sulfuric acid treated halloysite (1M acid, at 50 ̊C for 30 hrs). Reprinted with permission from Ref. 21 Copyright 2012 American Chemical Society Having different chemistries of inner and outer walls (aluminum oxide being inside and silicone dioxide outside) halloysite lumen has been effectively etched by sulfuric acid treatment. Enlarged lumen provides more space for active agents and hence improves loading efficiency. As much as 30 wt% of DGEBA (epoxy monomer) has been encapsulated by halloysite after 72 hrs treatment of the sample with 0.5 M sulfuric acid, threefold increase compared to untreated halloysite. Higher concentrations of acid yielded even more enhanced loading[3,22] . Similar observation has been done for benzotriazole[21] . 4. Industrial applications Having high surface area, elongated tubular shape with lumen which can serve as nanocontainer, and being environmentally friendly, halloysite mineral can be extensively used in the industry. Unlike other nano-tubular materials (such as BN, silica or carbon nanotubes) halloysite is a readily available cheap material, which makes it attractive for many technological applications. It belongs to the family of the clay minerals and can substitute kaolinite, montmorillonite, and bentonite as additives in composites. Particularly in paper production, modification of wood fibers by halloysite nanotubes were proven to increase the brightness and porosity of the paper sheets [23,24] . In ceramic industry halloysite was mainly used in tableware market as an addition to bone china and porcelain products [7] . Halloysite also has several current and potential applications in the field of nanotechnology. Several companies in United States, New Zealand and Turkey (such as Applied Minerals and Imerys) are supplying halloysite materials for industry. Halloysite nanotubes can be applied in the following major areas: 1)Polymer composites. 2) Protective coatings. 3) Concrete and cements. 4) Heterogeneous catalysts. 4.1 Polymer composites
  5. 5. 5     Polymer-clay nanocomposites allow increased tensile and flexural strength [25] , improved thermal resistance and flame retardancy [26-28] , decreased gas and liquid permeability and increased biodegradability [25] . Two most commonly used fillers for polymer nanocomposites are platy clays such as montmorillonite, hectorite and saponite [25] , and carbon based fillers such as carbon fibers, nanotubes and graphene [29,30] . Halloysite nanotubes have some advantages over carbon based fillers: they are naturally available, much cheaper than carbon nanotubes. Besides, they are not toxic. Even though halloysite nanotubes are less tough than carbon nanotubes, they still provide significant improvement in polymer tensile strength (5-10 wt % addition often doubles the composite strength) [26-28] . Halloysite also have certain advantages over platy clay minerals. Platy clay sheets are strongly stacked to each other which requires difficult and costly exfoliation process to obtain good dispersion within polymer matrix [25] . Halloysite has significantly less surface hydroxyl groups, capable of forming hydrogen bonded stacks or aggregates, and its tubule shape prevents tight particle stacking. Halloysite readily dispersed in many polymers without any surface treatments (e.g., polyamides, polyethylene tereftalates, epoxy based polymers, and polysaccharides) [31] . Halloysite improved fracture behavior and tensile properties of epoxy nanocomposites (Fig.7.). Critical stress intensity factors increased from 0.9 to 1.4 MPa• m1/2 after adding 10 % of halloysite [32,33] . Yield strength of the halloysite-ethylene propylene diene monomer (EPDM) composite increased ten times (12.9 MPa vs 1.3 MPa for neat polymer) while elongation at break increased threefold. Both SEM and TEM revealed good distribution of the tubes within polymer matrix even at the halloysite loading levels of 50 % [28] . Similar results were obtained on nanocomposites of halloysite with styrene-butadiene rubber (SBR) [34] . Figure 7. SEM images of halloysite epoxy composites. Reprinted with permission from Ref. 32 Copyright 2009 Elsevier Halloysite has formed effective network for the preservation of shape and structural integrity in phase change materials. Introducing 45 % halloysite into paraffin wax, a commonly used phase change material for thermal energy storage, preserved composite shape upon complete melting in 95 ̊C oven (Fig.8b). On the contrary original wax is completely spread within petridish and shape was totally lost above 72 ̊C, which is the melting point (Fig. 8a). This is related to the entangled network formed by halloysite nanotubes within paraffin matrix. Molten wax stay attached to the halloysite network and does not flow similar to the sponge filled with water. This new composite can be considered a promising heat storage material due to good thermal stability, high thermal/electricity conductivity and ability to preserve its shape during phase transitions. Figure 8. (a) Original paraffin wax and (b) 50% halloysite-wax composite stored at 95 ̊C.
  6. 6. 6     Halloysite nanotubes have been effectively utilized as nucleating agents for several polymers like polyethylene, isotactic polypropylene[35,36] , polybutylene terephthalate[37] and polyamide 6[38] . Unlike other clay fillers halloysite does not require organic surface modifiers like quarternary amine salts or organosilanes that lower the surface energy. The surface energy is believed to be responsible for the formation of crystalline spherulites as shown in Figure 9. Non isothermal peak temperature of crystallization was increased from 110.7 to 119.6 ⁰C while crystallization half time reduced from 0.525 to 0.458 min. upon addition of 30% halloysite within isotactic polypropylene at 20 ⁰C/min cooling rate (determined by DSC using Avrami-Jeziorni method). Activation energy of crystallization increased from 206.2 to 267.3 kJ/moles, which is associated with confinement effect of halloysite restricting polymer chain movements[36] . Similar effects have been reported for polyamide 6 and polybutylene terephthalate composites[37,38] . On the contrary organo modified montmorrilionite did not show any crystal nucleation due to the reduction of surface energy by surface treatment[37] . Nucleation capability in combination with fine dispersion, higher nanotube tensile strength and interaction with polymer chains is believed to be main reason behind tensile and impact strength improvement, while enhanced nucleation also significantly reduces the cycle time for extruded polymer parts. Figure 9. The final crystal morphology of pure polypropylene and polypropylene – 10% halloysite composites crystallized at 128 ⁰C. Reprinted with permission from Ref. 35 Copyright 2007 Elsevier. Halloysite polymer composites show increased thermal stability and flame retardancy compared to neat polymers. Addition of 10% halloysite increased the maximum weight loss temperature of polypropylene from 351 ˚C to 425 ˚C. Cone calorimetric analysis showed that 10% halloysite-PP composite generated twice less smoke compared to neat polymer and required 50% more time for ignition [39] . Similar observations were also reported on halloysite-polyamide[40] and halloysite-epoxy[41] nanocomposites. Improved resistance towards thermal decomposition was attributed to encapsulation of decomposition products by halloysite empty lumen and barrier properties of the tubes in the polymer matrix. 4.2 Protective coatings Another perspective application of halloysite nanotubes is paint industry. Current paint products contain a lot of additives on the basis of nanoparticles such as titanium dioxide (rutile), silica, clay, mica, latex, etc. Some of these nanoparticles are added to improve the properties of the paint while others are added just for the reduction of the product price [42,43] . Halloysite nanotubes offer great promise for the paint industry because these particles are readily miscible with big variety of the coatings and significantly improve their mechanical properties.
  7. 7. 7     Figure10. Mechanical properties of pure paint and halloysite paint nanocomposite. (a) Stress-strain relationship, and images of the (b) pure alkyd paint and (c) paint with 10% halloysite after rapid deformation test. Reprinted with permission from Ref. 13 Copyright 2011 American Chemical Society. (d) SEM image of microcrack on halloysite epoxy composite coating. Reprinted with permission from Ref. 33 Copyright 2007 Elsevier In Fig. 10a, stress-strain relationship of the oil based blue paint and halloysite paint composite is shown. Addition of 10% of the halloysite significantly improved the tensile strength of the paint. On the other hand halloysite paint composite also shows much better performance towards rapid deformation as compared to pure paint as demonstrated in Fig. 10 (b) and (c). A 0.2 kg metal bar was dropped to the painted iron plates (ASTM A366 alloy) from the height of 1 m in order to subject plates to the rapid deformations. The thickness of the metal plate was 1 mm in both cases. As it is indicated in the picture, metal coated with pure paint has a lot of cracks on it due to rapid deformation of the plate, while the same paint containing 10% halloysite did not show any evidences of the cracking[44] . This is referred to dissipation of the impact energy by clay nanotube de-bonding (pull-out) and prevented crack propagation due to bridging of microcracks[33] . Halloysite empty lumen can be loaded with corrosion inhibitors to achieve sustained slow release. Being compatible with polymer paints, loaded halloysite tubes are readily miscible with metal coatings. Earlier it was mentioned that addition of 10% halloysite into paints significantly improves their tensile properties. In addition to all these benefits, due to its empty lumen, halloysite nanotubes can be utilized to entrap corrosion inhibitors, which may improve anticorrosive metal coatings [11,12] . Figure11. Copper strips coated with (a) pure paint and (b) paint-halloysite nanocomposite. Reprinted with permission from Ref. 45 Copyright 2013 American Chemical Society Corrosion resistance study on the painted copper strips (Fig.11) revealed the increased corrosion inhibition performance of the coatings on the basis of the paint-halloysite composites. Strips were exposed to highly corrosive liquid containing 30 g/L of NaCl, for 6 months. As it is seen from the image significant amount of green corrosion
  8. 8. 8     products were accumulated underneath the paint on strips coated with ordinary paint while strip coated with halloysite-paint composite shows no evidence of corrosion [45] . The reason for the increased corrosion resistance of this coating is the slow release of the corrosion inhibitor entrapped into the hollow lumen of the tubes. Corrosion inhibitor slowly releases to the corrosive media once the paint is damaged causing the healing of the damage. Halloysite nanotubes also significantly improved the anticorrosive performance of the sol-gel coatings. Shchukin et. al. showed that tubules loaded with various organic corrosion inhibitors such as benzotriazole and 8- hydroxyquinoline significantly improves the anticorrosion property of the coatings for 2024 Aluminum alloy [12,46,47] . (A) (B) Figure 12. Corrosion current densities from metal strips made from 2024 Al alloy. Strips were coated with usual sol- gel coating (A), and with sol-gel containing halloysite (B). Reprinted with permission from Ref. 47 Copyright 2008 American Chemical Society In Fig. 12 localized corrosion current densities on coated and artificially scratched metal strips are shown. Corrosion current densities were measured by using Scanning Vibrating Electrode Technique, a method that uses vibrating PtIr electrode to detect corrosion current densities at certain locations of metal surface. The activity of the anodic corrosion current at the scratch of the pure sol-gel film was very high and rapidly increased during several hours, which is the indication of fast pitting corrosion process. However, in the case of modified sol-gel coating with halloysite nanotubes corrosion process was significantly reduced. No corrosion current was detected over 24 hours and this clearly indicates that incorporation of inhibitor loaded halloysite tubes into sol-gel coating significantly reduced the rate of corrosion process [12,47] . 4.3 Concrete and cement Concrete has many advantages over other building materials due to being moldable, easily engineered and rather cheap material. High performance concrete with good workability and enhanced strength, is prepared by incorporation of clay minerals and chemical additives within cement. Chemical composition and physical properties of halloysite nanoclay is effective in enhancing the performance of cementitious composites. During hydration, halloysite react with cement resulting in improved mechanical properties. It is active as pozzolan during hydration due to dissociation, yielding ions leading to the formation of cementitious compounds[48] . It is suggested that the dissociated alumina is consumed to cementitious minerals (calcium aluminosilicates). Also, released silica from the surface of halloysite react calcium ion to form hydrated calcium silicates[49] . Nanosize of particles provides improved reactivity towards hydration process. A study on concrete formulation based on type I Portland cement with 5% silica fume and up to 3% untreated halloysite revealed the increase in compression strength up to 24%, while flowability reduced by 65% [50] . In another study addition of 15% thermally treated halloysite within porous lightweight concrete prepared from Portland cement and expanded vermiculite resulted in over 100% increase in compression strength [51] . Incorporation of nanoclay resulted in higher formation of calcium silicate monohydrate, more consumption of calcium hydroxide and denser microstructures. Halloysite fill up the voids and lead to denser microstructure of mortar compared to the control sample (without halloysite). Enhanced consumption of calcium hydroxide to form CaSiO3·H2O is associated with external SiO2 on halloysite. Swelling of clay due to the entrapment of water within its layers leads to its expansion and filling capillary pores. The entrapped water gradually releases and cause further hydration and increase both strength and density. Elongated tubular morphology of halloysite provides additional improvement in strength due to the cross linking effect in cement matrix (Fig. 13).
  9. 9. 9     Figure 13. Scanning Electron Micrograph of the concrete prepared by type I Portland cement; control sample (left) and with halloysite nanoclay (right). Bridging of gaps within concrete by halloysite nanotube is clearly visible. Reprinted with permission from Ref. 50 Copyright 2013 Elsevier Rabehi et al studied the pozzolanic activity of Algerian halloysite treated at various temperatures in lime solution using X-ray diffraction and concluded that the clay calcinated at 750 ⁰C has remarkably high reactivity. Over 85% of the calcium hydroxide has been reacted with halloysite to form calcium silicate and aluminate which is responsible for the formation of high strength concrete[52] . Similar data has also been reported by Kang et al on halloysite samples from Korea. Incorporation of 10% calcinated halloysite improved resistance towards sulfuric acid etching by 32% while chloride ion diffusion was reduced over three times[51] . This is associated with reduced micro porosity and enhanced consumption of calcium hydroxide during pozzolanic reaction. In our recent research and development work with geopolymers, we demonstrated “smart” composites doped with halloysite. Admixing these clay nanotubes into fly ash geopolymers at 5-6 wt % provided sustained release of active agents resulting in longer term antibacterial protection without losing this concrete strength. Synergy in the improvement of mechanical properties, ability to adjust setting time and simplicity in geopolymers modification is an attractive feature of the work. 4.4 Heterogenous catalysts Halloysite have been effectively utilized as catalysts for synthesis of biofuels, hydrocarbon conversion and catalyst support for several chemical processes. Blending halloysites with commercial Y-zeolites provided low cost alternative for the production of mesoporous cracking catalysts with low environmental footprint. Having larger pores (10 – 20 nm in diameter) with extremely reactive Al+ sites, halloysite provide cracking of heavier petroleum components and faster desorption of reaction products from active sites. Hydroxyl groups existing in inner octahedral alumina surfaces of halloysite create extremely reactive sites with Hammett acidity below -8.2 (by acidity compares with 90 % sulfuric acid solution) [53] . A hydrocarbon conversion catalyst has been developed using acid treated and MgO doped halloysite clay. This catalyst showed higher performance towards the cracking of heavy hydrocarbons as compared with other natural clays (like kaolin and montmorillonite) [54] . Over 1 million tons of pure halloysite was mined during 1947 – 1976 years only from Dragon mine in Tintic district of Utah to meet the growing demands of the petrochemical industry [55] . Pyrolysis of the vacuum gasoil and its mixture containing 10% cottonseed oil have been studied in fluid catalytic cracking microreactor to determine the halloysite effect on activity and selectivity of the commercial Y- zeolite cracking catalysts. Addition of halloysite increased the depth of conversion by 7-10%, which contributed to the additional increase of the gasoline by 3-5% and light petroleum gases by 4-6%. Coke yield has been reduced by 7 - 15% (Fig. 14). This is associated with the faster desorption of reaction products from nanotube mesopores. Linear diffusion path from the pores of larger diameter allows rapid removal of lighter hydrocarbons from the surface of the acidic sites, preventing their deep dehydroxylation and hence reducing coke formation[56] .
  10. 10. 10     Figure 14. Pyrolysis of cottonseed oil and vacuum gasoil mixture on various catalysts. Variation of (a) gasoline yield, (b) conversion, (c) coke formation and (d) gas yield by temperature. Isomerization within halloysite pores occured more efficiently, causing more branched hydrocarbons in produced gasoline. Preferential formation of alkyl benzenes (vs benzene) upon halloysite addition is also indicative of enhanced chain branching, as branched olefins are precursors of the benzene derivatives. This is associated with better adsorption efficiency of fatty acid chains within halloysite aluminol surfaces compared to zeolite. Indeed, Raman spectroscopic study has shown that halloysite alumina surfaces (inner pores) have high tendency towards carboxylic acids adsorption [57] . Figure 15. Iron (a), cobalt (b) and silver (c) nanoparticles synthesized on halloysite nanotubes. Reprinted with permission from Ref. 3. Copyright 2013 Royal Society of Chemistry. Halloysite has been utilized as templates for metallic cobalt and iron nanoparticles for Fischer-Tropsch synthesis of artificial fuels from syngas (Fig.15). Cobalt supported catalysts were prepared by double solvent impregnation method using water-hexane mixture followed by calcination at 350 ⁰C. Fischer-Tropsch synthesis was conducted on prepared catalyst at CO/H2 ratio of 2, gas space velocity of 4 mL·hr-1 ·g-1 , 1 MPa pressure and 230 ⁰C. Carbon monoxide conversion exceeded 12% while C5+ selectivity was over 78%. Halloysite have been found to be efficient in preventing migration and agglomeration of cobalt nanoparticles during reaction, which is a critical factor in increasing the produced gasoline yield[58] . FeNi nanoparticles have been synthesized on halloysite nanotubes by immersing halloysite within aqueous solution of iron and nickel nitrates followed by calcination at 500 ⁰C and reduction with KBH4 as well as hydrogen gas. Nanoparticles had 2-4 nm diameter and uniformly distributed on halloysite external surface. External electron rich Si-O and hydroxide groups chelating iron and nickel oxides were thought to be responsible effective stabilization of nanoparticles on halloysite surface, preventing their aggregation. Catalyst treated by hydrogen gas
  11. 11. 11     was extremely efficient in decomposing phosphine gas into yellow phosphorus, resulting in 99% conversion at atmospheric pressure, 420 ⁰C and PH3 space velocity of 2520 mL/(hr*g)[59] . A catalyst has been prepared for reduction of aromatic nitro compounds to corresponding amines using silver supported halloysite nanotubes. Silver was synthesized from AgNO3 with aqueous solution of polyvinyl pyrrolidone and ethylene glycol and deposited on halloysite surface using mercaptoacetic acid as chelating agent. Effective immobilization of silver using thermal decomposition of silver acetate was also demonstrated (Fig. 16c). 10 nm size silver nanoparticles were uniformly coated on halloysite surface and found to be efficient catalyst in reduction of nitrophenol to the corresponding aminophenol[60] . 5. Food and Agricultural applications Very few studies have been performed to explore the potential of halloysite nanotubes in agricultural and food applications. Potential exists in utilizing halloysite as containers for encapsulation and controlled release of herbicides, pesticides and antibiotics to provide long lasting effect. Controlled release of several herbicides including organic trichloro derivatives, isothiazoline, chlorothanolil and pesticides malathion, spectricide and rotenone have been patented in United States[61] . Another perspective application of halloysite was found in preparation of custom biohybrid composites and paper for food packaging. Papers have been prepared by modification of lignocellulose wood fibers with halloysite clay and polyelectrolytes using Layer by Layer nanoassembly method (Fig. 16a,b). Porosity was found to increase by twofold with deposition of two layers of halloysite on wood fibers. Increase in porosity was observed for SiO2 and TiO2 modified papers as well though to a lesser extent (Fig. 16c)[62] . The increase of the paper porosity enhances the escape of ethylene and other gaseous products that would otherwise cause food rotting. Figure 16. Scanning Electron micrographs from the surface of the lignocellulose wood fibers (a) before and (b) after coating with four layers of halloysite nanotubes. (c) Results of the porosity test (time needed to pass 100 cm3 of air through handsheets) on paper prepared from halloysite, SiO2 and TiO2 modified wood fibers. Reprinted with permission from Ref. 62 Copyright 2007 Elsevier Addition of 4 % halloysite increased tensile strength of the halloysite-gelatin composites by 50%, while elastic modulus increased by threefold. This indicates strong chemical interaction between gelatin and halloysites, resulting in restricted polymer chain movements. Nanocomposites became 30% more permeable to water vapor[63] . Enhanced water permeability is desired in food packaging, where moisture induced rotting is greatly reduced.
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